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@fesso 69 9922? H. W. FSHER ET AL CONSTRUCTION OF CABLES Original FiledJuly 2, 1920 JUE.,

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` /A/VENTQ/as w. ww w mmm l/V/T/VESSES yie Patented if 6, i927..

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T OFF! Cv HENRY W. FISHER AND RALPH W. ATKINSON, OF PERTH .AMBOY, NEWJERSEY, AS- SIGNOBS TO STANDARD UNDERGROUND CABLE COMPANY, 0F?ITTSIBUBGH, PENN- YLVANA, A CBPORATION OF PENNSYLVNIA,

CNSTBUCTIN UF GABLEH.

@riginal application med July 2, 1920, Serial No. 393,534. Divided-andthis application niet?. Hay M', i924. Serial No. Wii.

ln Letters Patent of the United States, No. 1,524,124, granted J an. 27,1925, we have described and claimed a certain cable structure adaptedfor use for the transmission of electric power. The cable structure ofthe patent contains this informing idea: to put under pressure, andordinarily under heavy pressure, such .air as may accidentali be found,or may designedly be present, within the insulation of the cableconductor. Cables of the character with which we have to do, are cableswhich are required to carry currents under pressure of ten thousandvolts and upward, and our invention, which centers in the maintenanceunder pressure of air within the cable structure, has in view theproduction of an insulating envelope which shall excel insulatingenvelopes heretofore employed, both in the matter of dielectricstrength, and in the matter of security against leakage.

@ur present application is a divisional application, derived from ourpatent'mentioned above, and the invention as we shall herein describeand claim it, is found in a specic employment of the general idea.

Electric cables of the class with which we have to do consistsessentially` of conductor,

insulation, and sheath. There may be' a single conductor within a cable,made up of a single strand or of many strands, or there may be aplurality of conductors. Ordinarily there are three. These conductors'are enveloped in insulation; this insulation may be massive (indiarubber, for example), ap-

plied in plastic condition; again, this insu-` lation may consistfundamentally of a succession of solid bodies such-as rings of porcelainstrung upon the conductors; but most commonly the insulation for highvoltage cables is fundamentally composed of a body of fibrous material.rlhis fibrous material is ordinarily' paper or cloth i'nj strip formWrapped spirally and in superposed layers upon eachl conductorindividually; and if there be a plurality of conductors within a .singlesheath, the individually wrapped conductors are assembled symmetricallyabout an axis, the spaces filled and the whole grouping built out tocylindrical 'formby coarser space-filling-insulating substance (Juteordinarily or paper), andthe .whole cylindrical body is then ordinarilyWrapped about with another spiral wrapping, called the belt insulation,similar to that which encircles the individual conductors. A Sheath oflead incases and encloses the whole. Sometimes the fibrous strip or tapeof paper or cloth is.

applied in its dry or natural condition;

sometimes when applied it has already been filled and coated with aninsulating varnish;

sometimes it may, before or at the time of l on may be filled or coatedwith such an insulating compound; or, again, after the cable has beenotherwise so far completed as to be ready for the lead covering,-or,sometimes, at an earlier stage in the assembly, the entire vcable may beimpregnated with such an insulating compound, by any of the severalmethods known to the art.

ln the accompanying drawings, Fig. l shows in cross section a cable ofsuitable character, to have our invention applied to it; Figs. lll andlll are similar views, illustrating modincations in structure.

Referring, first, to Fig. lf, a typical threeconductor cable will befound illustrated. The individualv conductors are indicated at l; theseare here shown to be solid conductors, but the showin is'in this respectrather diagrammatic, and it will be understood that these conductors maybe, and ordinarily they will be, stranded conductors, composed each of abundle of relativelyne wires. However, in some cases, solid conductorswill be used. About each conductor lies a wrappedon envelo e 2 ofinsulation. 3 is the central filler, ordinarily of jute; and 4 are thelaterals, of the same material. The whole is wrapped about withv thebelt insulation 5, and 6 is the lead sheath.

As has been explained, the bodies of insulation 2 and 5 are, ordinarily,built up of paper (or cloth) tape, wrapped on spirally and in superposedlayers. This wrapped-on material (we are speaking in generalizations, toinclude common variations in cablemaking) may be cambric, varnished anddried before application; it may be cloth or paper, previously saturatedwith insulating titi compound; or it may be dry or unlled paper orcloth. Sometimes, though not neeessarily, the Wrapped-on material(particularly if previously saturated) is, at the time of Wrapping,coated or flooded With insulatieg compound. The ordinary paper-insa.lated cable is made by Wrapping the individual conductor l Withthe-paper insulation 2 While the paper is still in dry or unlledcondition. The separately enveloped conductors are then assembled withthe jute filling material 3, 4, and the belt insulation 5 is applied, inlike manner as insulation 2. At one or more stages in this process thecable is dried, and its insulation is impregnated by immersion ininsulating compound. The jute fillers 3 and 4L may or may not bepreliminarily impregnated; but, ordinarily, in the building of paperinsulated cables, these fillers are impregnated when (as usually occurs)the otherwise completed cable, ready for the lead sheath, is saturatedby immersion in the compound.

lin practicing our generic invention, a procedure may be adopted which,otherwise, would not commend itself; the Whole body oic insulation maybe applied and built up, without any insulating compound Whatever; thefibrous or porous material may be lett quite dry. lndeed, it may in somecases be additionally preferable to omit the illers 3 and 4. Thespecific inventionof this application, however, involves impregnation.

ln-the application of massive insulation, such as india rubber, in thefilling of fibrous or porous insulation With insulating compound, iniilling With liquid or viscid substance the spaces between blocks ofsolid insulation,--in all ot these cases it will inevitably be truethat, however carefully the Work be done, there Will remain, not Whollydislodged by, but incorporated Within the body of the insulation,bubbles (ordinarily minute) of air or other gas or Water vapor. Suchentrapped bubbles are points of Weakness; their presence augmentsdielectric losses When the cable is in service, with consequentinjurious heating of the cable; and, furthermore, their presence limitsthe voltage-carrying capacity of the cable; for by these bubbles asstepping stones disruptive electrostatic discharges of electricity maketheir Way from conductor to sheath, unless the operating voltage of thecable be limited to so small a value that this elect canne take place. f

The point to Which our explanatory statement now comes is that, in theinsulation of all cables, bodies of air are present; ordinarily thebodies of air are unintended, undesired, and to the utmost degreepossible (though never completely) eliminated. Our invention proceedsupon the acceptance oit the presence of air (or other gas) asinevitable, and so-deals with it that it shall be relatively ineffectiveas a source of Weak ness. Indeed, it becomes itself in certain casest-he etlective dielectric. It follows that carefulness in the exclusionof air is not in the same degree necessary; and indeed, as we have justintimated, We may with advantage apply our invention to the air-filledor dry paper-insulated cable-a cable otherwise inadequate for ordinaryhigh-power Work. The air-filled cable is specifically claimed in theparent patent. The specific invention of this application concerns thecable Whose insulation includes liquid or viscid substance. Ourinvention consists 'fundamentally in putting the air present in thecable insulation (Whether the quantity be reduced to the smallestpossible unintended remnant, or whether it be the much greater intendedquantity present in the dry or air-filled cable) under pressure, under apressure of substantial amount. By substantial amount We mean a pressureso great that fluctuations due to changes oit' temperature underoperative conditions (a range orp 10()o F., more or less) shall berelatively insignificant, in comparison with the actual value of thepressure. Practically, We find an absolute pressure of two atmospheres,(that is to say, one at mosphere above atmospheric) and upward adequateto effect the ends We have in view. Ordinarily the pressure used is muchhigher than two atmospheres, and ranges from 40 to 100 pounds to thesquare inch.

lt it be a matter of small occluded bubbles ot air or other gas, thesubstance in which the bubbles are held must, of course, at the timeWhen the pressure is applied, be responsive to the pressure. lt'normally that substance be viscid or solid, it may for the purposes ofour invention be rendered temporarily luidmordinarily by heat.

Pressure has tivo effects: in the case ot occluded bubbles it reducestheir size, and so is the equivalent of more complete elimination; butin all cases, that et the dry or air-filled cable as well, compressionof the air or other gas decreases in marked degree dielectric leakage athigh voltages and consequent heating, andincreases in marked degreedielectric strength. In this break'- ing down of insulation thephenomenon of ionization of the pocketed air takes place, What is termedinternal corona effect is brought about. The so segregated bodies oi airare, under the stress ol high voltage physically changed, so that theybecome elec trically conductive.

Where a conductor carrying a. high-voltage current is sepa-rated 'fromanother conductor body by an intervening air gap, the air is under astress which increases as the voltage of the passing current increaes.There is a critical point in this rise of voltage beyond which, if thepoint be exceeded, ionization oi' air in the gap will take place,

lll

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titl) matteo' venting more extended use ont insulated cables. We haveproceeded along a course Widely variant )fromy what hitherto has beenuniversally held to be good and proper practice; we may cease to striveto the yuttermost for the removal ot air bubbles, or tor the preventionci their occurrence; we may accept their presence as practicallyinevitable; and we so deal with them as to render them innocuous and nolonger sources ot danger to the cable when in service. -lindeed, we fmfr that, as one application ot our method, we can dispense with uidinsulating compound altogether; that we can build a. ry ca le, and makeit ed'ective. By pressure we so far raim the critical point of thecontained air that within the intended ranges of voltage fthe criticalpoint will not be reached, there will `be no ionization of the airwithin the body ot insulation, and no consequent breakdown, but theinsulation will be good and edective.

While we may employ an air-filled fibreinsulated cable for the practiceof our invention (specifically claimed in the parent case), it stillwill be borne in mind (as has already been explained) that the inventionis applicable to a cable more or less cornpletely lilled with insulatingcompound, to a cable insulated with massive insulation applied in liquidor plastic form, and to a cable whose insulation consists fundamentallyot bloclrs or solid insulation with filledor unlled spaces between-it isapplicable Wherever the body ot insulation contains air or other gas,whether in large and intended quantities, or in small, undesired,but'ineradicable quantities.

lit the insulation contains the air or gas in the form of occludedbubbles, and it the insulation be such `in nature as to be substantiallysolid under service conditions, it will be understood that the highpressure applied to the insulation while liquid, in order to bring aboutthe essential condition ot our invention, need not be continuallyniaintained alter the insulation has solid- 1 e llilor special uses,where the exactions are exceptional, it will be advantageous to build upabout the conductor an insulation which,

in those parts nearest the conductor, where stress 1s greatest, iscomposed of such material as rubber or varmshed cambrie, and

in the outer and remoter parts consists ot dry paper lled with air underpressure. rllhe characteristic feature oi insulation so built up will bethat it consists of superposed layers, the underlying layers relativelymassive and the outer layers relatively open and lled with air underpressure, rllhis arrangement will accomplish quite et'ectively the endsoit gradingi-much more edsctively, indeed, than can otherwise beaccomplished. Furthermore, the pneumatic pressure in the cable-will beoi additional advantage in increasing the strength ot the inner bodiesot varnished cloth or rubber in the same way as the dielectric strengthof a cable composed entirely oli these materials would be increased bythe use ot the pneumatic pressure. Such a graded cable has higherdielectric strength than a cable coni sisting wholly of either ont thetwo kinds of insulation under consideration, and in particular caseswill be itar more useful than either. f

The structure just described is illustrated in lFig. lill of thedrawings which, in view of what has already been said, will beimmediatelyand fully understood: l is the conductor, solid or stranded;2 is relatively massive insulation, suchas rubber, or it may bevarnished cloth; 2b is relatively porous insulation, such as dry paper;and 6 is the cable sheath. rllhe insulatin layer 2b will be understoodto be filled wit air (or other gas) and the gas to be maintained underpressure, as already described. The condition of bubbles of air in thelayer 2a has been suiiiciently dwelt upon.

@rdinarily in the practice ot our inveny the pressure will be maintainedin service by providing an air pump and an accumulator for compressedair (together with means for cleaning and drying the air). Communicationwill be maintained between the accumulator and the interior oil thecable. Suitable check valves will be provided, to prevent accidentalescape of air from the cable. Pressure maybe distributed throughout thelength oil the `cable by running a pressure supply pipe parallel withthe cable (either within the cable sheath or outside) and providingports of communication at suitable intervals.

When, as is herein specifically contemplated, the insulation consists ofor is filled with liquid, pressure may be maintained upon it, tocompress occluded bubbles, in

lllli such manner as We have indicated, or the requisite pressure may bemaintained by bringing a reservoir ot liquid into communi- Y of tin orother metal alloy. In common practice antimony, to an amount as great as1%, or tin, up to about 3%, is used for this purpose. ln some cases itwill be found necessary or desirable to use a pressure much higher thancan be withstood by a. sheath of alloyed lead. lin suchv case the sheathmay be wrapped with steel or bronze tape. Such a wrapping is indicatedat 17, Fig. HI. Since even a galvanized steel tape may be eatengradually away by chemical or electrolytic action, it will in some casesbe desirable to apply over such a wrapping of steel tape a second leadsheath. Again, the lead sheath of the cable may be relieved of pressurefrom within tending to distend it, by placing the lead sheathed cablewithin an air-tight pipe or conduit and maintaining elevated I pressurenot only within the sheathed cable but also in the space external ltothe cable and between it and the inner wall of the pipe. Such adaptationof material is of course to be made, wherever the user desires.

The primary advantage of our invention lies in the fact that thedielectric strength of the insulation will be considerably greater thanthat of cable insulation hitherto used. Thus it becomes apparent that itis possible to operate a cable of given dimensions (of given amount ofmaterial and requiring a given amount of duct space) at a voltage fiftyto one hundred per cent greater than is possible with the cables in useup to this time. yll`he importance of this advantage is far greater atextra high voltage than even at the voltage hitherto commonly used; for,if it were possible to remove from the cable hitherto used thelimitation respecting dielectric strength (that is to say, if theinsulation would not break down under great strain and be disrupted anddestroyed), still the dielectric losses of the kind already mentioned asleakage would become so great as to forbid operation at voltages greatlyeX- ceeding the normal.

Over against the advantages stated must be set the expense (in mostcases requisite) of maintaining the necessary pneumatic-pressure. Thisis a disadvantage or inconvenience which is relatively greater in thecase of short cable lengths than long; and it is relatively greaterwhere the amounts of power carried are small than where they are large.Furthermore, if the pressure required be so great as to approximate say100 pounds per square inch, the cable (this has been intimated already)and the terminals may have to be specially constructed to withstand thepressure.

In the case of important cables, carrying lar e amounts of power overgreat distances, suc special arrangements and construction of terminalsas may be found requisite will be of little moment when compared withthe cost and importance of the cable itself. Again, since the practiceof our invention permits transmission under voltages fifty to onehundred per cent greater than when a cable of the hitherto prevalenttype is employed, two cables of our invention can do the work of threeor four of equal size of the former type. Such a saving, manifestly,warrants increase of cost of cable and of terminals too, through a verywide margin.

Still another instance, where the advantages of our improved cableoutweiv'h the disadvantages, is found in case a sing e-con ductor cableand, to limited extent, the same is true ol a. three-conductor cable) ofrelatively short length connected to an open overhead line. Such a cableso connected may be of unusual importance, considering its length. lnthis case the advantages consequent on the use of our invention mayeasily justify the added expense. lf the short cable is to be used at a.station in which terminate other cables of our inven tion, the economyof the use of the invention in the short cable will be still moreclearly apparent.

lV here voltages are relatively low our invention will not, except underpeculiar circumstances be advantageous. Peculiar circumstances, such aswe have just suggested, may, for instance, be found where temperaturesare high and the possibilities for heat dissipation limited. But theprincipal tield of usefulness of our invention is found with cablesoperating under load of 20,000 volts i and upwards. `Without enlargementof dimensions, the voltage limit may, by the adoption of our invention,be increased from 20,000 to 35,000; from 25,000 to ll0,000, or more. Ur,if increase in voltage is not desired, thinner insulation may be used,with economy of space, or with advantageous enlargement of the conductorwithin. ln either case, the cost per unit of transmitting power would beless.

The possibility of increasing the operating voltage beyond the limitpermissible for a cable of the type hitherto employed opens an entirelynew field; it now becomes possible to transmit electric energy by cableeconomically over much greater distances; for the distance economicallypossible is, within a wide range, proportionate to the voltage. Forinstance, where the distance is so great as to forbid the use of cablesof types hitherto used, and to require instead the use of overheadtransmission, it may now become good practice, by means of ourinvention, to resort instead to cable transmission. VVhat has just beensaid was said with multiple-conductor cables in mind; similar conditionsobtain with respect to' single-conductor cables, although the voltagespermissible with single-conductor cables would be possibly fifty to onehundred per :mamon p cent greater than with multiple-conductor cables.lt is apparent, then, that a singleconductor cablev embodying ourpresent invention may be employed for carrying current under voltagesheretofore possible for open-wire overhead transmission only. 'lhat isto say, in certain cases transmission by the single-conductor cable ofour invention is possible, where previously cable trans1nission has notheretofore been possible by any means.

Sight must not be lost of the fact that when a cable of the presentinvention is applied in place of' av cable of precedent type, evenwithout any change in dimensions, and without change in voltage ofoperation, still the advantage will be enjoyed of diminished dielectricloss.

In the case of single conductor' cables, designed for very highvoltages, advantage will be found in the use of a hollow conductor; 'aconductor composed of a multiplicity of wires laid concentrically overthe surface of a. flexible conduit (made up ordina-rily of a helix ofmetal which is very Hexible, and which constitutes the core of thecable). The walls of such a central conduit are quite pervious to air,and, accordingly, by its very construction such a conduit affordsopportunity for free pneumatic communication to the farthest extent ofthe installation. inasmuch as the layer of wires which in this caseconstitutes the conductor is enclosed in an envelope of considerablethickness of wrapped-on paper insulation, and inasmuch as change in airpressure can become effective but slowly through such a layer of paper,it will be found that the maintenance of pressure in the hollow centerof the cable will be effective, to pre- -vent serious breakdown of thecable consemaintained near the conductor, where it is most needed. Thisfeature'of the invention is illustrated in Fig. II of the drawings. rlhecentral fiexible pervious conduit is indicated at 'i'. The conductor isindicated at 1, and is in this case made up of a plurality of strands ofsmaller wire. 2 is the wrappedon insulation enveloping the conductor,and 6 is the cable sheath.

In the case of a high voltage Lcable, economy demands that the cable beoperated at the highest voltage which the dielectric will safelywithstand; or, stated conversely, for a given working voltage thethinnest insulation which will safely withstand the stress will beapplied to the conductor. lli/ith this controlling condition in mind,the value of our invention will be fully apparent.

`We claim as our invention:

In preparing for high-voltage service an electric cable which includes aconductor insulated within an envelope containing bubbles of air in amass of material normally solid but liqueiiable by heating, the methodherein described which consists in placing the insulating envelope underpressure Vwhile at a temperature exceeding the point of liqueication ofthe material mentioned above, and, while maintaining the pressure,allowing the envelope to cool to a point lower than such point ofliquefaction.

.ln testimony whereof we have hereunto set our hands.

HENRY W. FlSHER. RALPH W.. ATKINSON.

